1. Field of the Invention
The present invention relates to a light or optical transmission system which performs a light repeating (relaying) transmission with optical repeaters or repeating installations including an optical direct amplification, and to a monitor control of optical repeaters used for the light transmission system.
In particular, the present invention is available for a system such as an optical submarine transmission system networked with a submarine repeater which amplifies an attenuated optical signal in a transmission line and a submarine branching installation where the transmission line is branched in the submarine to branch the signal into plural terminal equipments.
2. Description of the Related Art
FIG. 14 shows one example of a conventional light transmission system which has an optical repeater. In this example, a terminal equipment A is connected to a terminal equipment B through a light transmission line 3 into which "n" repeating installations 1-1.about.1-n are inserted. In each of the repeating installations 1-1.about.1-n component repeaters 1-1a-1-na including optical amplifiers are provided in the upstream direction and repeaters 1-1b.about.1-nb including optical amplifiers are provided in the downstream direction.
These light amplifiers perform an optical direct amplification of an optical signal inputted from the upstream side. Recently, a lot of optical fiber amplifiers are used as a light amplifier performing the optical direct amplification. This optical fiber amplifier is generally known as the one in which the optical signal is inputted to the optical fiber with excitation light from an excitation light source put in an optical fiber doped with rare-earth element.
To specify the respective repeating installations 1-1.about.1-n respectively, proper repeating installation numbers (for instance, "1" to "n") are allocated.
This prior art light transmission system is arranged such that the light of a single wavelength is transmitted with respect to a single optical fiber from the transmitting to the receiving terminal equipment.
Also, in preparation for the case where the repeating installation and the branching installation are failed, a monitor control function is provided to deal with the failure by rapidly specifying the failed portion in such a light transmission system.
For instance, in order to observe operating conditions of the current, the excitation power, the repeating input power, and the output power etc. of an excitation laser diode (not shown) in the repeating installations 1.about.3, the optical main signal on the down line is amplitude-modulated with a supervisory control signal 1 having digitally modulated information of the proper number "3" and the command number of the repeating installations 1.about.3 from the land terminal equipment A.
The repeating installations 1-1.about.1-n receive this AM signal, demodulate the supervisory control signal 1, and detect the proper number of the repeating installation. Only the repeating installations 1.about.3 designated by the proper number "3" respond to the supervisory control signal 1. The optical main signal on the up line is amplitude-modulated with the own operating condition information (monitor signal) based on the command number, and transferred to the termnial equipment A as a response signal 2.
The terminal equipment A receives this response signal 2 and will recognize the operating condition of the repeating installations 1.about.3. Similarly, the terminal equipment A can transfer the operating condition information of the repeating installation back to the own equipment by designating the proper number of other repeating installations.
Meanwhile, as the quantity of data transmission increases recently, the wavelength division multiplexing WDM technology in which optical signals of more than two waves are multiplexed into a single optical fiber at a terminal equipment is being utilized.
In this technology, the above-mentioned supervisory control signal is superimposed to the optical main signal at not only a specific wavelength but also all wavelengths. If the supervisory control signal is superimposed only at the specific wavelength, the modulation factor of the signal of the wavelength will be largely changed.
The purpose of this measure is to prevent the main signal of the wavelength from being deteriorated than that of the other wavelengths due to the supervisory control signal.
In this case, there has been proposed an optical branching and repeating transmission taking advantage of the difference of each wavelength, as shown by the dotted line in FIG. 14.
The branching installation in a system for optical branching and repeating operations using the wavelength division multiplexing technology is shown in FIG. 15. While this branching installation is shown in FIG. 14 in the form of a submarine branching installation 2 (ADM-BU) of a wavelength division multiplexing type installed in a sea cable, it branch the optical signal of "m" waves among "n" waves in a single optical fiber 3a transmitted from the terminal equipment A to a single optical fiber 3b in a branch device 4 in FIG. 15 and transmits the branched signal to a terminal equipment C. Moreover, an optical coupler 5 couples or synthesizes the optical signals of "m" waves in an optical fiber 3c from the terminal equipment C and of the "n-m" waves of the remainder not branched in the branch device 4. The coupled signal is transmitted to a terminal equipment B through a single optical fiber 3d.
The prior art in FIG. 15 shows an example where the wave number "n" is 8 and "m" is 7. The supervisory control signal 1 shown by .star-solid. sign is inputted to the branch device 4 in the form included in the optical main signal consisting of 8 waves from the terminal equipment A.
The 1st to 7th waves are branched to the terminal equipment C. The 1st to 7th waves transmitted from the terminal equipment C and the 8th wave from the branch device 4 are coupled in the optical coupler 5. Being included only in the 8 wave, the supervisory control signal will be transmitted to the terminal equipment B in the form in which the power of the signal light is decreased to 1/8.
Namely, when the supervisory control signal 1 passes through the branching installation 2a, the SN ratio thereof will be decreased.
Thus, upon the optical branching and repeating transmission performed, the SN ratio of the supervisory control signal which passes through the branching installation 2 or 2a is decreased as the branched wave number increases. It is required to enhance the power of the supervisory control signal 1 transmitted from the terminal equipment A "eight" times in the example of FIG. 15 in order to exactly receive and demodulate the signal 1 at a latter repeating installation 1, and to identify the proper number and the command number of the repeating installation.
However, such a system where the power of the supervisory control signal 1 is enhanced in the terminal equipment A, the modulation factor of the supervisory control signal 1 with respect to the optical main signal should be almost eight so that the transmission quality of the optical main signal is damaged, resulting in that this system can not be available.